The conventional semiconductor laser drive circuits may be classified roughly into the non-bias type and the bias type.
The non-bias type uses a laser driving method which sets the bias current of a semiconductor laser to 0, and drives the semiconductor laser by the pulsed current corresponding to the input signal.
On the other hand, the bias type uses a laser driving method which sets the bias current of the semiconductor laser to an oscillation threshold current of the semiconductor laser, adds the pulsed current corresponding to the input signal, to the bias current, and drives the semiconductor laser by a sum of the pulsed current and the bias current, while outputting the bias current to the semiconductor laser invariably.
When driving a semiconductor laser having a large oscillation threshold current by the above-mentioned non-bias method, even if the drive current corresponding to the input signal is supplied to the semiconductor laser, it will require a certain amount of time until the time the carrier signal having the concentration that enables the laser oscillation is created. The semiconductor laser in this case will emit light with a delay time.
There is no problem when the input signal is sufficiently large when compared with the light emission delay time, so that the amount of light emission delay can be negligible. However, when it is desired to drive the semiconductor laser at high speed in connection with the laser printer, the optical disk drive, the digital copier, etc., there is the problem that only the light emission time of the semiconductor laser that is shorter than the desired light emission time can be obtained.
For this reason, in order to make the light emission delay time of the semiconductor laser small, the bias type laser drive method in which the oscillation threshold current is variably supplied to the semiconductor laser as the bias current has been proposed.
In the bias type, the oscillation threshold current is supplied to the semiconductor laser, in advance, and it is possible to eliminate the light emission delay time as in the non-bias type.
However, even when the semiconductor laser is turned off, the semiconductor laser always emits light in a very small amount near at the oscillation threshold level (usually, 200 μW to 300 μW). In the case of optical communication, the extinction ratio becomes small due to the bias current, and in the case of the laser printer and the digital copier, etc., background stain occurs in the image formation due to the bias current.
For this reason, in the field of optical communication, it is proposed that the non-bias type is used fundamentally and the conventional semiconductor laser driving device is configured such that the oscillation threshold current is supplied to the semiconductor laser just before causing the semiconductor laser to emit light.
For example, Japanese Laid-Open Patent Application No. 4-283978 discloses a non-bias type semiconductor laser driving method for use in the field of optical communication. Japanese Laid-Open Patent Application No. 9-83050 discloses a non-bias type semiconductor laser driving method for use in the field of optical communication.
However, in the case of the laser printer, the digital copier, etc., in order to attain high resolution image forming, the system using a 650 nm (red) semiconductor laser, a 400 nm (ultraviolet) semiconductor laser, etc. is now under development.
These semiconductor lasers have such characteristics that it takes much time to create the carrier signal having the concentration that enables the laser oscillation when compared with the semiconductor lasers of the conventional type with the wavelengths of 1.3 μm, 1.5 μm and 780 nm. Even if the above improved method is used, there is the problem that only the light emission time of the semiconductor laser that is shorter than the desired light emission time can be obtained.
To solve the problem, an improved laser driving method has been proposed. FIG. 13 shows a conventional semiconductor laser driving device using such improved method.
As shown in FIG. 13, the semiconductor laser driving device includes a voltage-current (V-I) conversion circuit 104, an initialization circuit 108, a control circuit 109, an emission current generating unit 110, and a bias current generating unit 111. A laser diode (LD) (called the laser) which emits light by the drive current Iop at the output of the emission current generating unit 110 is provided. The bias current generating unit 111 outputs the bias current Ibi to the laser invariably. The voltage-current conversion circuit 104 outputs the sample hold current Ish to the laser via the switching circuit 105 in order to obtain the desired amount of emission light of the laser. The emission current generating unit 111 includes a current-output type DAC (digital-to-analog converter) which outputs to the laser the emission current Idac needed for light emission of the laser responsive to the input signal (which is the digital data outputted from the initialization circuit 108).
In the semiconductor laser driving device of FIG. 13, a sum of the bias current Ibi, the sample hold current Ish, and the emission current Idac is supplied to the laser diode LD in order to obtain the desired amount of emission light of the laser diode LD.
The bias current Ibi is a small current in the amount of about 1 mA. The emission current Idac is generated by detecting luminescence characteristics of the laser diode LD through an initialization operation performed by the initialization circuit 108. The sample hold current Ish is generated through an APC (automatic power control) process such that the voltage Vpd, which is transformed by the conversion of the monitored current of a photodiode PD in response to the emission light of the laser diode (LD) using a variable resistance 112, accords with a predetermined reference voltage Vr.
FIG. 14 is a timing chart for explaining operation of the semiconductor laser driving device of FIG. 13.
As shown in FIG. 14, the emission input signal Si is externally supplied to the semiconductor laser driving device in order to perform the emission light control of the laser diode LD. The input signal Si is delayed by the control circuit 109 so that the emission ON signal Sa is obtained, and the emission ON signal Sa is outputted to the emission current generating unit 110.
The SW control signal Sb is outputted by the control circuit 109, and it is asserted at the same time the input signal Si is asserted, so that the SW control signal Sb is maintained in the asserted state until the emission ON signal Sa is negated.
The LD drive current Iop is the current which drives the laser diode LD, and it is composed of the sum of the emission current Idac, the sample hold current Ish, and the bias current Ibi. Only the bias current Ibi is supplied to the laser diode LD until the emission input signal Si is raised to the high level.
Immediately after the emission input signal Si is raised to the high level (it is asserted), the sample hold current Ish is added to the bias current Ibi, so that the sum of the sample hold current Ish and the bias current Ibi is supplied to the laser diode LD. The sample hold current Ish at this instant is equal to the current value indicating a difference between the oscillation threshold current Ith of the laser diode LD and the bias current Ibi.
When a predetermined period of time progresses after the emission input signal Si is raised to the high level, the asserted ON signal Sa causes the emission current generating unit 110 to add to the LD drive current Iop the emission current Idac indicating the current value In according to the digital data signal SD received from the initialization circuit 108. It is possible to obtain the desired amount of emission light from the laser diode LD by the supplied drive current Iop. Thus, the laser diode LD is activated by the bias current Ibi and the impedance is made small, and the response characteristic to the oscillation threshold current Ith is improved. The semiconductor laser driving device can provide the emission pulse having the desired pulse width for the laser diode LD.
FIG. 15 is a diagram for explaining the luminescence characteristics of laser diode LD in the semiconductor laser driving device of FIG. 13.
When the ambient temperature rises to a high temperature as shown in FIG. 15, the luminescence characteristics of laser diode LD will change and the oscillation threshold current Ith of the laser diode LD will increase greatly.
Furthermore, the differentiation efficiency becomes small, the current value In of the emission current Idac before the desired emission power output is obtained from the oscillation threshold current Ith increases because of the decrease of the differentiation efficiency.
It is necessary to increase the current value In of the emission current Idac at the time of high temperature.
In FIG. 15, (a) indicates respective current values when the initialization operation is performed at normal temperature, (b) indicates respective current values when the ambient temperature rises to the high temperature, and (c) indicates respective current values when the initialization operation is performed at the high temperature.
The initialization circuit 108 detects the luminescence characteristics of laser diode LD by performing the initialization operation at normal temperature, and outputs the emission current Idac, indicating the current value In according to the detected characteristics, to the emission current generating unit 110.
Suppose that the current value of the oscillation threshold current Ith of the laser diode LD at normal temperature is set to IthA, the current value of the emission current Idac obtained through the initialization operation at normal temperature is set to InA, and the current value of the sample hold current Ish at normal temperature is set to IshA. In this case, the condition IthA=IshA+Ibi is met.
When the ambient temperature rises to the high temperature after the initialization operation at normal temperature is performed, in order to obtain the desired amount of emission light from the laser diode LD, it is necessary to increase the sample hold current Ish to supplement an increase of the current value In of the emission current Idac caused by the decrease of the differentiation efficiency in accordance with the rise of the ambient temperature.
As indicated by (b) in FIG. 15, the sample hold current Ish is increased from IshA to IshAa. This is because the current value InA of the setup emission current Idac remains unchanged unless the initialization operation is again performed by the initialization circuit 108.
In this case, the APC process is performed by using the sample hold current Ish, and the sum (IshAa+Ibi) of the sample hold current Ish and the bias current Ibi exceeds the oscillation threshold current value IthB of the laser diode LD at the time of the high temperature.
As shown in FIG. 14, even if the emission current Idac is not supplied to the laser diode LD, the laser diode LD will emit light because of the excessively large drive current.
When such phenomenon of the laser diode LD takes place even for a short time, it becomes the cause of background stain in the image formation by the laser printer or the digital copier. Such undesired problem is continued until the initialization circuit 108 performs the initialization operation again at the high temperature as indicated by (c) in FIG. 15. In addition, due to the system restrictions, it is difficult that the initialization circuit 108 frequently performs the initialization operation according to temperature changes.